The present invention relates to an exhaust gas cleaner having a filter which traps fine particles in exhaust gases, more particularly to an exhaust gas cleaner having a filter which traps fine carbonaceous particles (hereinafter referred to as "particulate matter") in exhaust gases discharged from diesel engines, soot in exhaust gases from boilers, etc.
Since floating fine particles such as particulate matter in exhaust gases discharged from diesel engines, soot from boilers, etc., are one of the causes of environmental pollution, exhaust gases are often filtered to remove them. In this method, filters choked up with the accumulated fine particles after use for some period should be exchanged to new one, or the filters should be cleaned up by removing the fine particles by some methods.
Conventionally, filters made of heat-resistant ceramics such as cordierite or a metal wire mesh, etc., are used, and fine particles accumulated in the filters are burned away by some heat source such as a burner, an electric heater, etc. In this conventional method, however, the filters are most likely to be damaged or melted by some spot heating or so during the burning of fine particles. To avoid this problem, the amount of the fine particles accumulated in the filters, the concentration of oxygen in the exhaust gas, etc. should be controlled.
To reduce the amount of the accumulated fine particles in the filters by using a method of burning them automatically, it is necessary to detect a change in pressure of the exhaust gas and to operate the heat source which ignites the fine particles in the filter in accordance with the pressure change, because the change in pressure of the exhaust gas can be a measure of how much the fine particles are accumulated, in other words, how much the filters are choked up. However, if a detection level is set such that the heat source is operated at high pressure generated by a high speed operation of the engine, the heat source would be idle when the engine is operated at a low speed, resulting in the clogging of the filter. On the other hand, if a detection level is set such that the heat source is operated even at a low gas pressure, the heat source would be operated most time, leading to an excess energy loss. Particularly in the former case, a fuel efficiency of the engine is lowered.
To solve these problems, a method of cleaning filters for reuse was proposed (Japanese Patent Laid-Open No. 57-203812). In this method, filters made of insulating materials are provided with electrodes to cause an electric current to flow through the fine particles trapped in the filter, thereby burning the fine particles in the filters with heat generated by the current flow.
FIG. 5 shows a conventional exhaust gas cleaner based on the above principle. This exhaust gas cleaner comprises a porous filter 1 made of an insulating material with electrode rods 2 inserted into the filter 1. It is, however, difficult in this exhaust gas cleaner to burn up the trapped fine particles in all parts of the filter 1 completely by the current flow between the electrodes, leaving a part of the filter (shaded part in FIG. 5) clogged with the unburned fine particles. Increasing voltage applied between the electrodes to avoid partial clogging of the filter as mentioned above may cause a partial damage or fusion of the filter because of an excess spot current.
FIG. 6 is a schematic cross-sectional view showing another conventional exhaust gas cleaner. This exhaust gas cleaner has a filter 1 made of an insulating material and electrodes 2, 2, each of which is in contact with each opposing end side of the filter 1. In this exhaust gas cleaner, the filter 1 is likely to get clogged with fine particles near the inlet side (shaded part in FIG. 6) of the filter. Increasing voltage between the electrodes 2, 2 may also cause a damage of the filter because of an excess spot current in the filter.
FIG. 7 shows another conventional exhaust gas cleaner. This exhaust gas cleaner comprises (a) a cylindrical filter 1 having a plurality of holes 11 extending axially, and (b) two electrodes 12, 12, each of which is in contact with the filter at the opposite side. One end of each hole 11 is covered with either electrode 12. Thus, one hole 11 which is covered with the electrode 12 at the inlet side (the left side of the filter 1 in FIG. 7) is open at the other end (the outlet side) of the hole 11, and another hole 11 which is opened at the inlet side is covered with the other electrode 12 at the outlet side. On the inner wall of the holes 11, electrode wires 13 are placed. Once the length of the wires 13 in the holes 11 is chosen long enough to cover almost all region from the inlet side to the outlet side, clogging with the fine particles may be avoided. However, the electrode wires 13 in the holes 11 prevent the exhaust gas from smoothly flowing in the filter. Aside from that, fixing the wires 13 on the inner walls of the holes 11 is quite difficult.
Therefore, an exhaust gas cleaner which is free from these problems mentioned above has been sought.